The present invention relates to a gas turbine having a closed-circuit cooling system for one or more nozzle stages and particularly relates to a gas turbine having inserts for impingement-cooling of the nozzle vane walls and which inserts are sectional to facilitate installation into the nozzle vane cavities, with determinant impingement-cooling gaps between the inserts and the nozzle vane walls.
In advanced gas turbines, nozzle stages are often provided with a closed-circuit cooling system for cooling the nozzle vanes exposed to the hot gas path. For example, each nozzle vane may include a plurality of cavities extending between the outer and inner nozzle bands. Impingement-cooling inserts are provided in one or more cavities and a cooling medium such as steam is passed into the insert for flow through openings or apertures in the side walls of the insert for impingement-cooling the adjacent wall portions of the nozzle vane. An example of a closed-circuit steam-cooled nozzle for a gas turbine is disclosed in U.S. Pat. No. 5,743,708, of common assignee herewith, the disclosure of which is incorporated herein by reference.
Typically, the nozzle insert is a unitary body provided by an insert supplier and nominally sized for reception within the cavity of the nozzle vane. It will be appreciated that the insert is inserted into the vane cavity and provides an impingement gap between the interior wall of the nozzle and the wall of the insert. However, because of manufacturing tolerances involved with the nozzle cavity and the insert per se, as well as a need to be able to dispose the insert endwise into the nozzle cavity, variations from the designed impingement gap along the length of the insert and nozzle vane wall frequently occur. A variation in the impingement gap can, in turn, cause a significant change in the heat transfer between the nozzle vane walls and the cooling medium. For example, it has been found that a 0.010 inch variation in the gap from a nominal dimension can result in an approximate 13% reduction in heat transfer coefficient. Also, this percentage increases exponentially with further impingement gap variation. Further, installation of a unitary insert into the nozzle vane cavity is somewhat difficult, oftentimes requiring a custom fit, which requires hand bench standoffs individually formed and hence is increasingly costly. There is also a potential for low-cycle fatigue as a result of the variation in heat transfer coefficient caused by the varying impingement gap.
In accordance with a preferred embodiment of the present invention, there is provided apparatus and methods for facilitating the disposition of an insert in a nozzle vane cavity to achieve a designed impingement gap between the internal wall of the nozzle and the wall of the insert, minimize or eliminate potential low-cycle fatigue problems and facilitate installation. To accomplish this, an insert comprised of two elongated hollow insert bodies is provided for installation separately within the nozzle cavity. Each insert body includes a hollow sleeve open at one end for receiving a cooling medium and closed at its opposite end. Each insert body also includes an outer wall portion having apertures through which an impingement cooling medium flows to impingement-cool registering wall portions of the nozzle, the remaining walls of the insert body being closed and without apertures. The insert bodies are configured for side-by-side disposition within the nozzle vane cavity with the walls containing the apertures in registration with the opposed wall portions of the nozzle vane. Inner wall portions of the bodies, when installed in the vane cavity, are spaced one from the other. The open ends of the bodies are also configured for securement of the bodies to one another in situ, i.e., within the nozzle vane cavity after installation. Standoffs are provided on each of the insert body walls containing the apertures. One or more spreaders are provided between the inner walls of the insert bodies to flex the bodies outwardly to engage the standoffs against the wall surfaces of the nozzle vane.
To install the insert bodies, the bodies are inserted into the nozzle vane cavity separately, thereby eliminating manufacturing tolerance stackup. After insertion, one or more spreaders are installed and joined to the insert bodies to engage the standoffs against the internal nozzle cavity walls, thus positively determining the designed impingement gap. The open inlet ends of the inserts can then be secured to one another and to the nozzle. By employing this configuration and installation procedure, the designed impingement gap is provided between the side wall portions of the insert bodies and the nozzle vane walls throughout the length of the vane. By installing the insert to the correct impingement gap, heat transfer is significantly improved with corresponding benefit to improved low-cycle fatigue life.
In a preferred embodiment according to the present invention, there is provided an insert for a cavity of a nozzle vane of a gas turbine for impingement-cooling of the walls of the vane, comprising a pair of elongated hollow insert bodies disposable in side-by-side relation to one another within the cavity, the bodies having a plurality of apertures through oppositely directed outer walls thereof, inner wall portions of the bodies being spaced from one another and at least one spreader disposable between the inner wall portions for maintaining the inner wall portions of the insert bodies spaced from one another.
In a further preferred embodiment according to the present invention, there is provided a nozzle for a gas turbine, comprising a nozzle vane having a plurality of cavities extending between outer and inner ends of the body and spaced from one another between leading and trailing edges of the vane, an insert in one of the cavities including a pair of elongated, hollow insert bodies in side-by-side relation to one another for receiving a cooling medium, each body having a plurality of apertures through oppositely directed outer walls thereof for flowing the cooling medium to impingement-cool registering side wall portions of the vane, the bodies having respective inner wall portions spaced from one another and at least one spreader disposed between the inner wall portions to maintain the inner wall portions spaced from one another.
In a still further preferred embodiment according to the present invention, there is provided a method of installing a cooling medium insert into a cavity of a nozzle vane for a gas turbine wherein the insert includes a pair of discrete elongated hollow insert bodies, each having an outer wall portion with a plurality of apertures therethrough, comprising the steps of inserting the discrete insert bodies into the vane cavity for disposition therein in side-by-side relation to one another, with the outer wall portions thereof in registration with side wall portions of the vane and inserting a spreader between spaced inner wall portions of the insert bodies to maintain the outer wall portions of the bodies spaced a predetermined distance from the side wall portions of the vane.